The folks at HomeGain (they provide marketing tools to real estate pros) did a survey — FSBO vs. Realtor. They asked more than 1,000 homeowners about their experiences selling a home.

You can read the details here, but here are the basics:

Used a Realtor and were successful selling: 59%
Went FSBO and were successful selling: 39% (but of those, only 71% would go FSBO again)

Used a Realtor, were able to sell their home, and would use a Realtor again: 88%.
Used a Realtor, were unable to sell their home, but would use a Realtor again: 81%.

Went FSBO, couldn’t sell, turned to a Realtor: 24%.

The survey didn’t go into things like ask vs. sale price, which is probably good — the FSBO numbers are skewed because so many are done between people who know each other beforehand.

Once again, here’s a link to the HomeGain post.